1. Field of Invention
This invention relates to communications systems. Specifically, the present invention relates to multi-mode communications systems employing methods for synchronizing and controlling oscillators to facilitate handoff.
2. Description of the Related Art
Many wireless phones accommodate both a digital communications standard as well as an analog communications standard. Wireless phones that accommodate more than one wireless communications standards are multi-mode phones.
Many wireless phones integrate digital Code Division Multiple Access (CDMA) capabilities with analog Advanced Mobile Phone System (AMPS) capabilities. Incorporating both the digital CDMA and analog AMPS standards in one wireless phone expands effective user coverage area. For example, a user with a multi-mode phone incorporating both analog and digital systems can effectively communicate in areas lacking digital coverage.
As wireless communications technology progresses, so do the numbers of digital standards. Currently, the digital standards CDMA and Global System for Mobile communications (GSM) are in widespread use. Digital systems are often preferable to analog systems as they generally provide higher quality, higher capacity and improved security features.
Unfortunately, existing phones typically incorporate only one digital standard and, as a result, their coverage area is limited. Wireless phones incorporating multiple digital standards have been slow to develop due to various obstacles. For example, wireless digital communications systems often require oscillator synchronization to lock on to a received signal. For a wireless phone to efficiently handoff from one digital system to another, novel and unique oscillator synchronization methods must be employed to avoid significant delay and/or dropped calls during system handoff. In addition, oscillators employed in wireless communication systems may drift over time. Digital communications systems employing multiple oscillators must account for additional frequency drift associated with the additional oscillators to effectively receive signals
As international commerce and tourism expands, the demand for wireless phones that can accommodate more than one digital standard also increases. For example, currently people traveling between the United States and Europe, which operate under different digital wireless communications standards, must carry a separate phone for each standard. This is often expensive and inefficient.
Hence, a need exists in the art for multi-mode communications device that can efficiently accommodate more than one digital standard. There is a further need for a system and method for facilitating oscillator synchronization to enable efficient handoff between communications systems employing different digital standards.
The need in the art is addressed by the system for synchronizing a second receive chain relative to a first receive chain in a digital multi-mode communications system of the present invention. In the illustrative embodiment, the inventive system is adapted for use with CDMA and GSM digital communications system standards and includes a first circuit for determining a first frequency associated with the first receive chain. A second circuit adjusts a second frequency associated with the second receive chain based on the first frequency and provides a desired second frequency to the second receive chain in response thereto.
In a specific embodiment, the first circuit includes a first counter that receives a signal associated with an output of a first oscillator and provides a measurement of the first frequency in response thereto. The second circuit includes a frequency synthesizer for providing the desired second frequency in response to a frequency control or reference signal. The frequency synthesizer may be implemented as a phase-locked loop, a direct digital synthesizer, and/or a frequency-controllable oscillator. The second circuit also includes a circuit for generating the control or reference signal based on a difference between the second frequency and the first frequency. A second counter receives a signal characterized by the second frequency and provides the second frequency in response thereto.
In the preferred embodiment, the second circuit includes a circuit for generating the control or reference signal based on a difference between the second frequency and the first frequency, where the first frequency corresponds to a first count of the number of cycles occurring in a signal of the first frequency during a specific time interval, and where the second frequency corresponds to a second count of the number of cycles occurring in a signal of the second frequency during the specific time interval. The specific time interval represents the time required for the first counter to count to a desired first count. The desired second count is the number of cycles of the signal of the second frequency occurring during the specific time interval as counted by a second counter. The second circuit includes a subtractor for subtracting the second count from a desired second count, which provides an error signal in response thereto. The second circuit includes a frequency tuner for providing the control or reference signal to the frequency synthesizer or the first oscillator, where the control or reference signal is based on the error signal.
In an alternative embodiment, the difference between the second frequency and the first frequency is the difference between the ratio of the first frequency to the second frequency and a predetermined desired ratio of the first frequency to the second frequency.
The novel design of the present invention is facilitated by use of the second circuit that synchronizes the second frequency relative to the first frequency. The first frequency is already actively tuned based on received signals from a first digital communications system by pre-existing oscillator tuning circuitry. By employing the tuned first frequency and a desired known relationship between the first frequency and second frequency, the second circuit pre-tunes the second frequency. As a result, multi-mode communications systems, and in particular, digital multi-mode communications systems constructed in accordance with the teachings of the present invention can efficiently handle handoff between digital systems employing different reference oscillator frequencies.